A typical process for the aqueous dispersion polymerization of fluorinated monomer includes feeding fluorinated monomer to a heated reactor containing a fluorosurfactant and deionized water. Paraffin wax is employed in the reactor as a stabilizer for some polymerizations, e.g., polytetrafluoroethylene (PTFE) homopolymers. A free-radical initiator solution is employed and, as the polymerization proceeds, additional fluorinated monomer is added to maintain the pressure. A chain transfer agent is employed in the polymerization of some polymers, e.g., melt-processible TFE copolymers to control melt viscosity. After several hours, the feeds are stopped, the reactor is vented and purged with nitrogen, and the raw dispersion in the vessel is transferred to a cooling vessel.
For use in fluoropolymer coatings for metals, glass and fabric, polymer dispersion is typically transferred to a dispersion concentration operation which produces stabilized dispersions used as coatings. Certain grades of PTFE dispersion are made for the production of fine powder. For this use, the dispersion is coagulated, the aqueous medium is removed and the PTFE is dried to produce fine powder. Dispersions of melt-processible fluoropolymers for molding resin use are also coagulated and the coagulated polymer dried and then processed into a convenient form such as flake, chip or pellet for use in subsequent melt-processing operations.
As described in U.S. Pat. No. 3,391,099 to Punderson, dispersion polymerization involves two generally distinct phases. The initial period of the reaction is a nucleation phase in which a given number of polymerization sites or nuclei are established. Subsequently, there occurs a growth phase in which polymerization of fluorinated monomer on established particles occurs with little or no formation of new particles. Successful production of the high solids fluoropolymer dispersion generally requires the presence of the fluorosurfactant, especially in the later growth phase of polymerization in order to stabilize the dispersion preventing coagulation of the fluoropolymer particles.
Fluorosurfactants used in the polymerization are usually anionic, non-telogenic, soluble in water and stable to reaction conditions. The most widely used fluorosurfactants are perfluoroalkane carboxylic acids and salts as disclosed in U.S. Pat. No. 2,559,752 to Berry, specifically perfluorooctanoic acid and salts, often referred to as C8, and perfluorononanoic acid and salts, often referred to as C9. Because of recent environmental concerns with regard to perfluorooctanoic acid and salts, there is interest in reducing or eliminating perfluorooctanoic acid and its salts in fluoropolymer polymerization processes.
There has been a similar environmental concern with regard to perfluorooctane sulfonate (PFOS), an 8 carbon fluorosurfactant formerly sold by 3M as a stain repellent under the trademark Scotchguard®. For uses such as stain repellency, fluorosurfactants with short hydrophobic chain lengths, e.g., perfluorobutane sulfonate, have been used to replace perfluorooctane sulfonate. However, if attempts are made to employ short hydrophobic chain length fluorosurfactants in the commercial polymerization of fluoromonomers to achieve desirable solids concentrations, significant amounts of undispersed polymer (also referred to as coagulum) will form in the reactor. This coagulum typically must be discarded as waste. For example, the short chain fluoroether CF3CF2CF2OCF(CF3)COOH is disclosed in Example XII of U.S. Pat. No. 3,271,341 to Garrison as a polymerization surfactant for polytetrafluoroethylene (PTFE). However, in this example which produces 1243 grams of PTFE dispersion, 500 grams of coagulum is produced, i.e., 29% of the total weight of PTFE produced is coagulum.
Rather than employ short chain fluorosurfactants, other known methods employ higher molecular weight materials such as fluoropolyethers in the polymerization of fluoropolymers. U.S. Pat. No. 4,864,006 to Gianetti et al. discloses the polymerization of fluorinated monomers in the presence of a perfluoropolyether having neutral end groups, perfluoropolyether oil, which is used in the form of an aqueous microemulsion. The perfluoropolyether oil has molecular weight of at least about 500 and the aqueous microemulsion of the oil is prepared using a suitable surfactant which can be selected from known perfluorinated carboxylic or sulfonic acids or from perfluoropolyethers having one or two acid end groups. U.S. Pat. No. 6,395,848 to Morgan et al. discloses an improved process for the aqueous dispersion polymerization of fluorinated monomers using a combination of fluorosurfactant, which can be a fluoroalkyl carboxylic or sulfonic acid or salt thereof or fluoroalkoxy aryl sulfonic acid or salt thereof, and perfluoropolyether carboxylic or sulfonic acid or salt thereof. The perfluoropolyether carboxylic or sulfonic acid or salt thereof employed in the examples of Morgan et al. have molecular weights ranging from 2000 to 7500. The fluoroalkyl carboxylic or sulfonic acid surfactants disclosed in Morgan et al. are those commonly used in dispersion polymerization such as perfluorooctanoic acid or salts and other fluorosurfactants with the same or longer chain length which can be used alone as polymerization surfactants.